Semiconductor integrated circuits have reached so great a scale of integration that large scale integrated circuits (LSI) and very large scale integrated circuits (VLSI) are now used in practice. At the same time, the minimum pattern size of integrated circuits reaches the submicron region and will become finer. Micropatterning is generally carried out by lithography, for example, by forming a thin film on a substrate, coating a resist thereon, effecting selective exposure to form a latent image of the desired pattern, developing the resist to form a resist pattern, dry etching through the resist pattern as a mask, and removing the resist, leaving the desired pattern.
As the pattern feature size becomes finer, the light source used for exposure in the lithography undergoes a transition to shorter wavelength ones such as deep-UV, vacuum-ultraviolet (VUV), electron beams (EB) and x-rays. The latest stage of lithography considers to use as the exposure light source excimer lasers (e.g., KrF laser of wavelength 248 nm and ArF laser of wavelength 193 nm) and a F2 laser of wavelength 157 nm. These lasers are expected to be effective for micropatterning.
In resist compositions for forming submicron-size patterns using exposure light of shorter wavelength or in the vacuum UV region, a variety of polymers or copolymers are used. Proposed thus far are polymers or copolymers of acrylic esters or alpha-substituted acrylic esters having an adamantane skeleton and acid-eliminatable protective groups in the ester moiety (see JP-A 4-39665), polymers or copolymers of acrylic esters or alpha-substituted acrylic esters having a norbornane skeleton and acid-eliminatable protective groups in the ester moiety (see JP-A 5-257281), polymers or copolymers of cyclohexylmaleimide (see JP-A 5-257285), polymers having a cellulose skeleton in the backbone which undergoes cleavage with acid (see JP-A 6-342212), and polyvinyl alcohol and polyvinyl alcohol derivatives (see JP-A 7-333850).
However, these polymers and copolymers do not satisfy all the characteristics necessary as resist material including dry etching resistance, transparency to deep-UV, solubility in resist solvents, wettability with developers, adhesion to silicon and other substrates, and solubility in stripping agents. None of them are easy to synthesize. There is a need to have polymers satisfying the above requirements.
On the other hand, photoresist compositions comprising polymers based on aliphatic cyclic hydrocarbon as the backbone and having a cyclic skeleton bearing acid-decomposable functional groups are disclosed in WO 97/33198, JP-A 9-230595, JP-A 9-244247, and JP-A 10-254139. These cyclic polymers have good dry etching resistance and transparency to deep-UV, but leave unsolved the problems of dissolution in resist solvents in high concentrations, wettability with developers, and adhesion to silicon substrates.
To satisfy all the above-described performance factors, that is, the performance factors necessary as the base resin in positive photoresist compositions that have a good light transmittance, high sensitivity, high resolution, and high affinity to basic developers, and are capable of forming satisfactory patterns, so that the compositions are effective for semiconductor micropatterning using UV or deep UV (inclusive of excimer lasers), hydrogenated products of ring-opening metathesis polymers having a narrow molecular weight distribution were recently proposed. See JP-A 11-130843, JP-A 11-130844, JP-A 11-130845, JP-A 11-171982, JP-A 2000-109545, JP-A 2001-27803 and Japanese Patent Application No. 2000-113351.
On the other hand, the degree of integration of semiconductor LSI has increased at a rate as fast as about 4 folds every three years. The increased degree of integration is arrived at by improving the fineness of lithography. To this end, many micropatterning techniques have been proposed including ultrahigh resolution technique during exposure, thermal flow during development and during baking after resist pattern formation, and over-etching during etching. A typical method of forming micropatterns is described below.
A resist material is spin coated on a Si wafer, after which the resist coating is baked at an elevated temperature (referred to as prebaking or PB, hereinafter) and selectively exposed to light through a pattern mask. The exposed resist on the wafer is further baked at an elevated temperature (referred to as post-exposure baking or PEB, hereinafter) and developed with a developer to form a resist pattern. Using the thus formed pattern as a mask, wet or dry etching is conducted. The resist in which exposed areas become solubilized in the developer is designated positive working resist whereas the resist in which exposed areas become insolubilized is designated negative working resist. It is known that thermal flow after resist pattern formation is effective for further reduction of the diameter of contact holes (see J. S. Kim, J. C. Jung, G. Lee, M. H. Jung, K. H. Baik, J. Photopolym. Sci. Technol., 13, 471 (2000)).
Resist compositions comprising the aforementioned polymers or copolymers of acrylic esters or alpha-substituted acrylic esters having an adamantane skeleton and acid-eliminatable protective groups in the ester moiety fail to solve the problem of dry etching resistance even when the thermal flow process is introduced, and thus perform poorly as the micropatterning resist. Resist compositions comprising the aforementioned polymers based on aliphatic cyclic hydrocarbon as the backbone and having a cyclic skeleton bearing acid-decomposable functional groups do not thermally flow due to their high glass transition temperature, and inhibit the introduction of the thermal flow process.
On the other hand, techniques of combining different types of polymers as a base resin have been under study. However, when polymers of different backbone structures are combined, they are often incompatible and fail to form a homogeneous resist solution. Even when a resist solution is formed, phase separation or haze develops upon coating, failing to form a satisfactory film. Alternatively, even when a film is formed, partial stripping of the film can occur during pattern formation due to the uneven distribution of the base resin components.